Lexus IS Forum banner
1 - 20 of 32 Posts

·
Podiatrist
Joined
·
22,260 Posts
Discussion Starter · #1 ·
A simple question...
Many of the FI kits out there do not offer a fuel return line.

What benefits would you gain from having one?
I know that a FMU is attached inline to the fuel return line and adjust pressure, but I fail to see how this changes much.
 

·
Registered
Joined
·
37 Posts
fuel return

ok. our cars fuel system from the factory is return-less....so in order to properly add more fuel to motor for the extra power you will create w/ a turbo or supercharger you need to install a fuel press regulator which requires a return fuel line. Fuel Press regulators restrict the amount of fuel that returns to the tank therefore creating more or less pressure...
 

·
Podiatrist
Joined
·
22,260 Posts
Discussion Starter · #4 ·
but if you play with return pressure, you're still eventually gonna create some pressure on the fuel pump as well aren't you?

It's a circle.
 

·
Registered
Joined
·
634 Posts
How can a turbo kit like TBKOs or a supercharger kit like PLPs work without a fuel return line? I spoke w/ Jeff and he said that his kit doesn't require a fuel return line. However if I remember correctly his older kit comes w/o a fuel return line.
 

·
Registered
Joined
·
4,996 Posts
hmm Like he said our cars are a return less system(actually the return is in the tank)

If you want to install a FPR (AKA FMU) you have to run a line back to the tank. Remember as you boost you have pressure trying to force the fuel back in the injector so you want increase you fuel pressure to offset the boost.

I dont know about stress on the fuel pump, If you ask a fuel pump to flow more then its designed to it will die.
 

·
Registered
Joined
·
1,073 Posts
Yes IS300s as well as several other cars are returnless systems. An FPR (or FMU) raises fuel pressure in proportion to how much boost it sees. The higher the boost the higher the fuel pressure. The advantage of this is that you can squeeze more capacity out of your stock injectors.

You can raise fuel pressure by modding the in-tank regulator which is your static fuel pressure but hard to make it boost dependent. The way many turbo systems will work is either with additional injectors controlled by emanage for example or lengthen injector pulse widths and raise static fuel pressure but you can only run fairly low boost. That's why many of the higher end turbo systems run engine management and larger injectors like 720+CC injectors. Supras can use 550cc injectors for similar power levels since they can put an FMU in and raise fuel pressure dynamically. We can put return systems on our cars but have to tap the fuel rail and put a regulator, run a line back to the tank and get some fittings for the return

The only advantage of returnless systems is emissions ... you don't have fuel evaporating in the return. most of the tuners on this board have a return kit that you can get. I know for sure Toyomoto and TBKO do. I am sure PFS does too, not sure about others.
 

·
Podiatrist
Joined
·
22,260 Posts
Discussion Starter · #8 ·
turbo97se said:
Yes IS300s as well as several other cars are returnless systems. An FPR (or FMU) raises fuel pressure in proportion to how much boost it sees. The higher the boost the higher the fuel pressure. The advantage of this is that you can squeeze more capacity out of your stock injectors.

You can raise fuel pressure by modding the in-tank regulator which is your static fuel pressure but hard to make it boost dependent. The way many turbo systems will work is either with additional injectors controlled by emanage for example or lengthen injector pulse widths and raise static fuel pressure but you can only run fairly low boost. That's why many of the higher end turbo systems run engine management and larger injectors like 720+CC injectors. Supras can use 550cc injectors for similar power levels since they can put an FMU in and raise fuel pressure dynamically. We can put return systems on our cars but have to tap the fuel rail and put a regulator, run a line back to the tank and get some fittings for the return

The only advantage of returnless systems is emissions ... you don't have fuel evaporating in the return. most of the tuners on this board have a return kit that you can get. I know for sure Toyomoto and TBKO do. I am sure PFS does too, not sure about others.
TBKO doesn't. I think toyo, and PFS do. SRT does not.
 

·
Registered
Joined
·
769 Posts
I asked this same question a while ago and here's my understanding of it. Someone please correct my interpretation if I have something inaccurate.

With a returnless fuel system, fuel pressure always stays the same. We'll use 60psi just for kicks. When you throw a turbo or s/c on the car, you are effectively reducing the fuel pressure. So, if you normally run 15psi on your turbo, your effective fuel pressure becomes 45psi (60-45). If you run 30psi on the turbo, your effective fuel pressure becomes 30psi (60-30), etc. Reduced fuel line pressure reduces the amount of fuel being forced into the engine. Less fuel can quickly cause a lean condition and we all know what happens when the car runs lean.

A return fuel line system monitors the effective fuel pressure and increases the fuel pressure as necessary to maintain the 60psi. So, if your turbo is set at 30psi, it will up the fuel line pressure to 90psi to offset the turbo (90-30=60). This will keep your injectors pushing the same amount of fuel into the motor all of the time, there by compensating for what might normally be a lean condition.

I know PFS and Toyomoto have a return system. I'm having mine installed in the next week or so ;).
 

·
Registered
Joined
·
1,784 Posts
DarkStorm said:
I asked this same question a while ago and here's my understanding of it. Someone please correct my interpretation if I have something inaccurate.

With a returnless fuel system, fuel pressure always stays the same. We'll use 60psi just for kicks. When you throw a turbo or s/c on the car, you are effectively reducing the fuel pressure. So, if you normally run 15psi on your turbo, your effective fuel pressure becomes 45psi (60-45). If you run 30psi on the turbo, your effective fuel pressure becomes 30psi (60-30), etc. Reduced fuel line pressure reduces the amount of fuel being forced into the engine. Less fuel can quickly cause a lean condition and we all know what happens when the car runs lean.

A return fuel line system monitors the effective fuel pressure and increases the fuel pressure as necessary to maintain the 60psi. So, if your turbo is set at 30psi, it will up the fuel line pressure to 90psi to offset the turbo (90-30=60). This will keep your injectors pushing the same amount of fuel into the motor all of the time, there by compensating for what might normally be a lean condition.

I know PFS and Toyomoto have a return system. I'm having mine installed in the next week or so ;).

Darkstorm nailed it. Good explanation. Dots a plenty
 

·
Registered
Joined
·
1,784 Posts
SophieSleeps said:
TBKO doesn't. I think toyo, and PFS do. SRT does not.
SRT has one available if you want to add it to your stage 1 or 2, and it's standard on stages 3 and 4.

I'm pretty sure TBKO will also add a RRFPR and return line at your request.
 

·
Registered
Joined
·
7,294 Posts
I have the CarTech Rising rate FPR or FMU whatever you prefer. It is activated by the boost pressure in the manifold via a vacuum hose. I have this mounted to the fuel "feed" line and fuel rail on one side of it. It then has another hose that attached to my fuel "return" on the opposite side. What happens it that if my pump is pumping at 75PSI at idle. but only 50PSI is needed at that vacuum level.... it opens a spring/vacuum actuated bleeder valve internally that then ports the "extra" 25PSI flow of fuel out of the FMU back into the return line. Thus keeping a steady 50PSI feeding the injectors.

As Boost sets in and you need more fuel.... the FMU gets the boost signal from the intake manifold and doesnt allow the bleeder to open up as much.. thus increasing the fuels PSI level to the injectors and bleeding off less. (ie: of i need 65PSI of fuel at 7lbs of boost.... the FMU will open the bleeder return line valve just enough to bleed off 10PSI from the pumps 75PSI output... to give the injectors a steady 65PSI while at boost.

Its not too difficult to understand once you actually do this stuff tiral and error if with FPGauge and all that junk that I like to do. :)

De... Here's a rough pic i took over the summer.... I have sinced moved the FMU off the rail and mounted it on the inner fender well. but you get the idea ;)
 

·
Registered
Joined
·
3,305 Posts
there ya go de.. now I dont even have to explain it to ya... thanks BDR I did not want to type out both sides of the explanation...you can use a FPR and a FMU at the same time if needed....but if you got a good fuel setup and a good FPR you can go without a FMU...
 

·
Podiatrist
Joined
·
22,260 Posts
Discussion Starter · #15 ·
Still a little confused.

So say the stock fuel line runs at a normal 50psi.
You are now turboed the fock up at say 20psi. 50-20 = 30psi.

But why does that make sense. If I blow something into a hose at 20psi...and something on the other end is blowing against me, that doesn't decrease the pressure in the hose. It increases it.


Also, how does the return line and FPR increase the line pressure?
I picture the system like this.
1. Fuel pump shoots fuel through fuel line towards injectors
2. Injectors cum in the engine
3. Return line and FPR run back to the tank

So how does the FPR regulate pressure in the fuel line...before the injectors?
 

·
Registered
Joined
·
634 Posts
he acutally recommended to order 6 bigger injectors instead of the additional one if someone wanted to run 1 bar.

going4boost said:
his additional injectors are HUGE, id assume large enough to compensate..

hchan said:
I talked to Jeff the other day and he said that even if someone wants to run 1 bar, his kit does not need a fuel return line. How is he doing it w/o a fuel return line?
 
1 - 20 of 32 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top